It is well known that drying of humid fuel, such as biomass fuel, produces essential advantages in connection with the subsequent combustion of the fuel. Separate drier systems for e.g. biomass fuel have been constructed. Drying heat is then produced, for instance in a separate furnace, wherein part of the moist fuel, is consumed. Flue gases from the oven are used to dry the fuel, for instance in a rotary drier. The moist flue gases from the drier may be condensed for recovery of heat that may be used to pre-dry the fuel to a certain extent before the latter is fed to the drier.
It is likewise known to utilise flue gas condensation in connection with conventional combustion of moist fuel, for instance in a hot water boiler. In this case, the high moisture contents obtained in the flue gases due to the moisture in the fuel are utilised.